What should I do if the downstairs decoration knocks on the wall, causing serious cracks in my bathroom?

Hello, the responsibility for this problem lies entirely with the construction party downstairs. I suggest you consult the client first! If negotiation fails to solve the problem, you can complain directly to the Consumers Association! Seek specific damages! If the Consumers Association can't handle it, there are two ways. One is the legal way, you can sue in court! The second is the way of the media, which can be exposed through the media! Sometimes the power of public opinion should not be underestimated! I hope your problem can be properly solved!